The present invention relates to an improved susceptor which inhibits the deposition of process gasses on the edge and backside of a substrate, which may be more easily removed and cleaned, and which is adapted for use with JMF type wafers.
Chemical vapor deposition (CVD) is one of a number of processes used to deposit thin films of material on semiconductor substrates. To process substrates using CVD, a vacuum chamber is provided with a susceptor configured to receive a substrate. In a typical CVD chamber, the substrate is placed into and removed from the chamber by a robot blade and is supported by a substrate support during processing. A precursor gas is charged into the vacuum chamber through a gas manifold plate situated above the substrate, where the substrate is heated to process temperatures, generally in the range of about 250xc2x0 to 650xc2x0 C. The precursor gas reacts on the heated substrate surface to deposit a thin layer thereon and to form volatile byproduct gases, which are pumped away through the chamber exhaust system.
A primary goal of substrate processing is to obtain the largest useful surface area, and as a result the greatest number of chips, possible from each substrate. This is highlighted by the recent demands from semiconductor chip manufacturers to minimize edge exclusion on the substrates processed, so that as little of the substrate surface as possible, including portions near the edge of the wafer, is wasted. Some important factors to consider include processing variables that affect the uniformity and thickness of the layer deposited on the substrate, and contaminants that may attach to the substrate and render all or a portion of the substrate defective or useless. Both of these factors should be controlled to maximize the useful surface area for each substrate processed.
One source of particle contamination in the chamber is material deposited at the edge or on the backside of the substrate that flakes off or peels off during a subsequent process. Substrate edges are typically beveled, making deposition difficult to control over these surfaces. Thus, deposition at substrate edges is typically nonuniform and, where metal is deposited, tends to adhere differently to a dielectric than to silicon. If a wafer dielectric layer does not extend to the bevel, metal may be deposited on a silicon bevel and eventually chip or flake, generating unwanted particles in the chamber. Additionally, chemical mechanical polishing is often used to smooth the surface of a substrate coated with tungsten or other metals. The act of polishing may cause any deposits on the edge and backside surfaces to flake and generate unwanted particles.
A number of approaches have been employed to control the deposition on the edge of the substrate during processing. One approach employs a shadow ring which essentially masks a portion of the perimeter of the substrate from the process gasses. One disadvantage with the shadow ring approach is that, by masking a portion of the substrate perimeter, the shadow ring reduces the overall useful surface area of the substrate. This problem is made worse if the shadow ring is not accurately aligned with the substrate, and alignment can be difficult to achieve.
Another approach employs a purge ring near the edge of the substrate for delivering a purge gas along the substrate edge to prevent edge deposition. The purge gas limits or prevents the deposition gas from reaching the substrate edges and thus limits or prevents deposition on the wafer beveled edge.
A wafer typically sits radially inside the purge ring, with a gap therebetween. Conventionally, purge rings are made of aluminum and are welded to the substrate support in an effort to prevent the ring from deforming during processing. However, during the thermal cycling which occurs within a CVD processing chamber, the aluminum rings nonetheless deform, losing the integrity of their shape and therefore compromise their ability to keep particles from depositing on the substrate edge. This can change the size of the gap, leading to non-uniformity of deposition across the wafer edge. As the aluminum rings expand and contract, material thereon can flake, and create particles which can contaminate the wafer. Furthermore, deformation of the aluminum ring tip or edges can cause the wafer to break.
Further, in order for the rings to work effectively for shadowing and/or for purging, they must be frequently cleaned to remove deposition material which can alter the gap or flake off and contaminate the wafer. Such cleaning increases chamber downtime, reduces throughput and results in higher operating costs.
Some wafers have a small notch in an outer edge thereof which is used to properly align the wafer with the substrate support and purge ring. Recently, the use of JMF type wafers presents additional problems in delivering a purged gas to the edge of the entire wafer. JMF type wafers typically have a flattened side, whereby the radius of the wafer to that flattened side is a small amount less than the radius of the wafer to the non-flattened portions. As a result, the standard purge rings do not properly align with the flattened edge of these JMF type wafers. The resulting effect is that the purge gas does not reach the flattened wafer edge. Hence, unwanted deposition of materials can occur on and near the fattened edge of the wafer, including JMF type wafers.
Accordingly a need exists for an improved susceptor which can reliably prevent edge deposition for all wafers, including JMF wafers, and which can be easily cleaned.
The present invention provides exemplary apparatus and methods for processing substrates, and more specifically, for ensuring purge gases reach the substrate edge, including edges of JMF type wafers, to help prevent unwanted deposition thereon.
One embodiment of the present invention provides an apparatus for processing substrates. The apparatus includes a chamber and a substrate support disposed in the chamber. An edge ring is disposed on the substrate support. The edge ring has a lip portion which at least partially overhangs an upper surface of the substrate support to define a gap between the lip portion and the upper surface. In this manner, the edge ring is designed to form a gap which properly directs purge gases to edges of the substrate, including JMF type substrates.
In one embodiment, the apparatus includes a purge gas system coupled to the gap for transferring a purge gas through the gap and under the lip portion. The purge gas can be a wide variety of gases, including argon and other inert gases.
In another embodiment, the lip portion has a generally flat inner edge that is adapted to match a substrate profile, for example, a JMF type substrate. In one embodiment, the lip portion extends over the upper surface a prescribed distance from an edge of the substrate support. In one embodiment, the prescribed distance is between about 1.5 mm and about 3.0 mm, and in another embodiment is about 2.0 mm. It will be appreciated by those skilled in the art that the prescribed distance will vary depending in part upon the substrate size (e.g., 200 mm, 300 mm, and the like), and shape.
In alternative embodiments, the edge ring comprises aluminum nitride (AlN), a ceramic (e.g., Al2O3), and the like. Similarly, in alternative embodiments, the substrate support comprises a ceramic heater, an aluminum heater, and the like.
In another embodiment, an apparatus for processing substrates includes a chamber, a substrate support disposed in the chamber, a first edge ring, such as a purge gas ring, disposed on the substrate support, and a second edge ring, such as a shadow ring, for mating engagement with the first edge ring. The second edge ring has a lip portion which at least partially overhangs the substrate support upper surface to define a gap therebetween. Hence, in this embodiment, the second edge ring helps define the appropriate gap.
The present invention further provides methods for supporting a substrate in a chamber. One such method includes providing an apparatus for processing substrates as described above, positioning a substrate on the support, and flowing a purge gas into the gap between the edge ring lip portion and the support upper surface so that the purge gas impinges upon the substrate edge.
In one aspect, the method further includes positioning a second edge ring above the edge ring to further define the gap. In another aspect, the method includes depositing a material on the substrate upper surface, with the purge gas flowing to substantially prevent deposition of the material on the substrate edge. The depositing step may comprise a CVD process, a PVD process or the like for depositing a metal layer, a dielectric layer, or the like.
In one embodiment, the method includes positioning the substrate so that a generally flat portion of the substrate edge is aligned with the lip portion or with the lip portion inner edge. Such a method is useful, for example, for processing JMF type substrates or other substrates having a generally flat edge portion.